Photocatalytic composition comprising metallic particles and two semiconductors, one of which is composed of cerium oxide

ABSTRACT

The invention relates to a composition that contains a first semiconductor SC1, particles that comprise one or more element(s) M in the metal state selected from among an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of the periodic table, and a second semiconductor SC2 that comprises cerium oxide, with said first semiconductor SC1 being in direct contact with said particles that comprise one or more element(s) M in the metal state, with said particles being in direct contact with said second semiconductor SC2 that comprises cerium oxide in such a way that the second semiconductor SC2 covers at least 50% of the surfaces of the particles that comprise one or more element(s) M in the metal state. The invention also relates to its preparation method as well as its application of photocatalysis.

The field of the invention is that of composite materials and theirapplication in photocatalysis. Composite material is defined as a solidthat consists of at least two compounds of different chemical natures.

Below, the groups of chemical elements are provided according to the CASclassification (CRC Handbook of Chemistry and Physics, Editor: CRCPress, Editor-in-Chief D. R. Lide, 81^(st) Edition, 2000-2001). Forexample, group VIII according to the CAS classification corresponds tothe metals of columns 8, 9 and 10 according to the new IUPACclassification.

PRIOR ART

The literature mentions examples of composite materials that containsemiconductors, in particular composite materials that consist ofcore-shell-type particles on the surface of a semiconductor substrate.This type of solid has been developed particularly in photocatalysisapplications.

C. Li et al. (J. Hydrogen Energy, 37, pp. 6431-6437, 2012) revealed thesynthesis of solids based on TiO₂ nanotubes on which particles ofmetallic copper oxidized on their surfaces are deposited byphoto-assist.

H. Lin et al. (Catal. Comm., 21, pp. 91-95, 2012) propose a compositethat is prepared by co-precipitation that consists of AgBr/Ag/AgI, withAgBr and AgI both being semiconductors.

By successive impregnations, C. Wang et al. (Chem. Eng. J., 237, pp.29-37, 2014) prepared a material that comprises contacts between WO₃ andPt, on the one hand, and Pt and TiO₂, on the other hand, prepared byco-precipitation.

Finally, H. Tada (Nature Materials, 5, pp. 782-786, 2006) proposes asolid based on hemispherical particles having a layer of CdS around anAu core, which particles are deposited on the TiO₂ semiconductor.

The object of the invention is to propose a composition that contains afirst semiconductor SC1, particles that comprise one or more element(s)M in the metal state selected from among an element of groups IVB, VB,VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of the periodic table, and asecond semiconductor SC2 that comprises cerium oxide, said firstsemiconductor SC1 being in direct contact with said particles thatcomprise one or more element(s) M in the metal state, with saidparticles being in direct contact with said second semiconductor SC2that comprises cerium oxide in such a way that the second semiconductorSC2 covers at least 50% of the surfaces of the particles that compriseone or more element(s) M in the metal state.

The use of cerium oxide that constitutes the semiconductor SC2 makes itpossible, surprisingly enough, to obtain a photocatalyst that hasphotocatalytic performances that are enhanced in relation to the knownphotocatalysts of the state of the art.

According to a preferred variant, the first semiconductor SC1 is also indirect contact with the second semiconductor SC2.

According to a preferred variant, said first semiconductor SC1 forms asubstrate, said substrate contains on its surface core-shell-typeparticles, with said shell being formed by said semiconductor SC2 thatcomprises cerium oxide, said core being formed by said particles thatcomprise one or more element(s) M in the metal state.

According to a variant, cerium oxide for the most part consists ofCe₂O₃.

According to a variant, the element M in the metal state is selectedfrom among platinum, palladium, gold, nickel, cobalt, ruthenium, silver,copper, rhenium, or rhodium.

According to a variant, the cerium oxide content of the semiconductorSC2, expressed in terms of the element Ce, is between 0.01 and 50% byweight in relation to the total weight of the composition.

According to a variant, the content of the element(s) M in the metalstate is between 0.001 and 20% by weight in relation to the total weightof the composition.

According to a variant, said particles that comprise one or moreelement(s) M in the metal state come in the form of particles of sizesof between 0.5 nm and 1000 nm.

According to a variant, the composition comes in the form of nanometricpowder.

According to a variant, the semiconductor SC1 is selected from amongTiO₂, Bi₂S₃, Bi₂O₃, Fe₂O₃, ZnO, WO₃, CuO, ZnFe₂O₄, MoS₂, and In(OH)₃.

According to a variant, the shell has a thickness of 1 nm to 1000 nm.

The invention also relates to its method for preparation comprising thefollowing steps:

-   -   a) A suspension that contains a first semiconductor SC1 in a        liquid mixture that consists of water and/or one or more organic        compounds and at least one metal precursor that is selected from        among an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, IIB,        IIIA, IVA and VA of the periodic table is prepared while being        stirred, and the suspension is irradiated by an irradiation        source such that at least a portion of the emission spectrum of        said source consists of photons having energies that exceed the        width of the forbidden band of the semiconductor SC1,    -   b) Then under stirring and irradiation of said irradiation        source, a soluble cerium precursor with a degree of oxidation of        +3 is introduced into the suspension that is obtained in step        a),    -   c) Then, under stirring and irradiation of said irradiation        source, a basic agent or acid agent is introduced in such a way        as to bring about the precipitation of cerium oxide,    -   d) Then, the composition is separated from the suspension of        step c),    -   e) The composition that is obtained in step d) is dried,    -   f) Optionally, the dried composition that is obtained in step e)        is subjected to a heat treatment.

According to a variant, the metal precursor is selected from among aprecursor of platinum, palladium, gold, nickel, cobalt, ruthenium,silver, copper, rhenium, or rhodium.

According to a variant, in step c), the pH is between 9 and 13 after thebasic agent or acid agent is introduced.

The invention also relates to the use of the composition according tothe invention or the composition that is prepared according to thepreparation method as a photocatalyst.

DETAILED DESCRIPTION OF THE INVENTION Composition According to theInvention

The invention relates to a composition that contains a firstsemiconductor SC1, particles that comprise one or more element(s) M inthe metal state that are selected from among an element of groups IVB,VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of the periodic table,and a second semiconductor SC2 that comprises cerium oxide, with saidfirst semiconductor SC1 being in direct contact with said particles thatcomprise one or more element(s) M in the metal state, said particlesbeing in direct contact with said second semiconductor SC2 thatcomprises cerium oxide in such a way that the second semiconductor SC2covers at least 50% of the surfaces of the particles that comprise oneor more element(s) M in the metal state.

In a preferred manner, the composition consists of a first semiconductorSC1, particles that comprise one or more element(s) M in the metal statethat are selected from among an element of groups IVB, VB, VIB, VIIB,VIIIB, IB, IIB, IIIA, IVA and VA of the periodic table, and a secondsemiconductor SC2 that comprises cerium oxide.

According to an important aspect of the invention, the firstsemiconductor SC1 is in direct contact with particles that comprise oneor more element(s) M in the metal state, with said particles being indirect contact with a second semiconductor SC2 that comprises ceriumoxide. Preferably, the first semiconductor SC1 is in addition in directcontact with the second semiconductor SC2.

According to a preferred variant of the invention, said firstsemiconductor SC1 forms a substrate; said substrate contains on itssurface core-shell-type particles, with said shell being formed by saidsemiconductor SC2 that comprises cerium oxide, said core being formed bysaid particles that comprise one or more element(s) M in the metalstate. The use of cerium oxide that constitutes the shell of thecore-shell-type particles on the surface of a semiconductor substrateSC1 according to the invention makes it possible, surprisingly enough,to obtain a photocatalyst that has enhanced photocatalytic performancesin relation to the photocatalysts known from the state of the art thatdo not contain the core-shell-type substrate structure.

The second semiconductor SC2 covers at least 50% of the surfaces of theparticles that comprise one or more element(s) M in the metal state, asurface in a preferred manner of greater than 60% and in a verypreferred manner of greater than 75%. The covering rate is measured byXPS (X-ray photoelectron spectrometry in English terminology), forexample on an ESCA KRATOS® Axis Ultra device with an Al monochromaticsource at 1486.6 eV, and a passage energy of 40 eV, and expresses thecovering of the total surface of the particles that comprise one or moreelement(s) M in the metal state.

The shell has a thickness of 1 nm to 1000 nm, preferably 1 nm to 500 nm,and in a particularly preferred manner 2 to 50 nm.

The composition contains a first semiconductor SC1. The semiconductorsSC1 that are used according to the invention comprise at least oneinorganic, organic, or organic-inorganic composite semiconductor. Thewidth of the forbidden band of the inorganic, organic ororganic-inorganic semiconductor is in general between 0.1 and 5.5 eV.

According to a first variant, the semiconductor SC1 comprises at leastone inorganic solid. The inorganic semiconductor can comprise one ormore of the elements that are selected from among the elements of groupIVA, such as silicon, germanium, silicon carbide or silicon-germanium.It can also consist of elements of groups IIIA and VA, such as GaP, GaN,InP, and InGaAs, or elements of groups IIB and VIA, such as CdS, ZnO,and ZnS, or elements of groups IB and VIIA, such as CuCl and AgBr, orelements of groups IVA and VIA, such as PbS, PbO, SnS, and PbSnTe, orelements of groups VA and VIA, such as Bi₂Te₃ and Bi₂O₃, or elements ofgroups IIB and VA, such as Cd₃P₂, Zn₃P₂, and Zn₃As₂, or elements ofgroups IB and VIA, such as CuO, Cu₂O, and Ag₂S, or elements of groupsVIII and VIA, such as CoO, PdO, Fe₂O₃, and NiO, or elements of groupsVIB and VIA, such as MoS₂ and WO₃, or elements of groups VB and VIA,such as V₂O₅ and Nb₂O₅, or elements of groups IVB and VIA, such as TiO₂and HfS₂, or elements of groups IIIA and VIA, such as In₂O₃, In₂S₃, orIn(OH)₃, or elements of groups VIA and lanthanides, such as Ce₂O₃,Pr₂O₃, Sm₂S₃, Tb₂S₃, and La₂S₃, or elements of groups VIA and actinides,such as UO₂ and UO₃. In a preferred manner, the semiconductor isselected from among TiO₂, Bi₂S₃, Bi₂O₃, CdO, Ce₂O₃, CeO₂, CoO, Cu₂O,Fe₂O₃, FeTiO₃, In₂O₃, In(OH)₃, NiO, PbO, ZnO, WO₃, CuO, ZnFe₂O₄, MoS₂,Ag2S, CdS, Ce₂S₃, Cu₂S, CuInS₂, In₂S₃, ZnFe₂O₃, ZnS and ZrS₂ andIn(OH)₃. In a very preferred manner, the semiconductor is selected fromamong TiO₂, Bi₂S₃, Bi₂O₃, Fe₂O₃, ZnO, WO₃, CuO, ZnFe₂O₄, MoS₂, andIn(OH)₃.

According to another variant, the semiconductor SC1 comprises at leastone organic semiconductor. Among the organic semiconductors, it ispossible to cite tetracene, anthracene, polythiophene, polystyrenesulfonate, phosphyrenes, and fullerenes.

According to another variant, the semiconductor SC1 comprises at leastone organic-inorganic semiconductor. Among the organic-inorganicsemiconductors, it is possible to cite the crystallized solids of theMOF type (for Metal Organic Frameworks in English terminology). The MOFsconsist of inorganic sub-units (transition metals, lanthanides . . . )and are connected to one another by organic ligands (carboxylates,phosphonates, imidazolates . . . ), thus defining crystallized,sometimes porous, hybrid networks.

The semiconductor SC1 can optionally be doped with one or more ions thatare selected from among metal ions, such as, for example, ions of V, Ni,Cr, Mo, Fe, Sn, Mn, Co, Re, Nb, Sb, La, Ce, Ta, Ti, non-metal ions, suchas, for example, C, N, S, F, P, or by a mixture of metal and non-metalions.

According to another variant, the semiconductor SC1 can be sensitized onits surface with all organic molecules that can absorb photons.

The semiconductor SC1 can come in different forms (nanometric powder,nano-objects that may or may not comprise cavities, . . . ) or shapings(films, monoliths, micron balls or millimeter balls, . . . ).

The composition contains a second semiconductor SC2. The semiconductorSC2 comprises cerium oxide. Preferably, the element cerium has oxidationdegree(s) +3 and/or +4. In a very preferred manner, the cerium oxide forthe most part consists of Ce₂O₃. “For the most part consists of Ce₂O₃”is defined as a Ce₂O₃ content that exceeds 50% by weight, preferablythat exceeds 60% by weight, and in a particularly preferred manner thatexceeds 70% of the total weight of the semiconductor SC2. Optionally,the semiconductor SC2 can in addition contain cerium hydroxides. Thesemiconductor SC2 preferably does not contain an element of the group ofmetals other than cerium.

The content of cerium oxide, expressed in terms of the element Ce, isbetween 0.01 and 50% by weight, preferably between 0.5 and 20% byweight, in relation to the total weight of the composition.

The composition comprises particles that comprise one or more element(s)M in the metal state that are selected from among an element of groupsIVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of the periodictable. Said particles that comprise one or more element(s) M are indirect contact with said semiconductors SC1 and SC2 respectively. Saidparticles can consist of a single element in the metal state or severalelements in the metal state that can form an alloy.

“Element in the metal state” is defined as an element that belongs tothe family of metals, with said element having oxidation degree zero(and therefore in metal form).

Preferably, the element or elements M in the metal state are selectedfrom among a metal element of groups VIIB, VIIIB, IB and IIB of theperiodic table, and in a particularly preferred manner from amongplatinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper,rhenium, or rhodium. Said particles that comprise one or more element(s)M in the metal state preferably come in the form of particles of sizesof between 0.5 nm and 1000 nm, in a very preferred manner between 0.5 nmand 100 nm.

The content of the element(s) M in the metal state is between 0.001 and20% by weight, in a preferred manner between 0.01 and 10% by weight, inrelation to the total weight of the composition.

The composition according to the invention can come in different forms(nanometric powder, nano-objects that may or may not comprise cavities,. . . ) or shapings (films, monoliths, micron balls or millimeter balls,. . . ). The composition according to the invention advantageously comesin the form of nanometric powder.

Preparation of the Solid

The composition according to the invention can be prepared according toany method that is known to one skilled in the art. According to anembodiment, the composition is obtained by photodeposition of the metalelement or elements M forming the metal particles (and therefore thecore in a core-shell substrate structure), and then by condensationinduced by precipitation while being irradiated by a cerium precursor ofoxidation degree +3 (forming the shell in a core-shell substratestructure) on a semiconductor SC1 (forming the substrate in a core-shellsubstrate structure) that contains the metal particles on its surface.

It should be noted that a preparation by the dry impregnation technique(in general seeking a high dispersion of the metal on the substrate) ofa copper precursor does not make it possible to obtain a compositionaccording to the invention in which the second semiconductor SC2 thatcomprises copper oxide covers at least 50% of the surfaces of theparticles that comprise one or more element(s) M in the metal state.

More particularly, the method for preparation of the compositionaccording to the invention comprises the following steps:

-   -   a) While being stirred, a suspension that contains a first        semiconductor SC1 in a liquid mixture that consists of water        and/or one or more organic compounds and at least one metal        precursor that is selected from among an element of groups IVB,        VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of the periodic        table is prepared, and the suspension is irradiated by an        irradiation source such that at least a portion of the emission        spectrum of said source consists of photons having energies that        exceed the width of the forbidden band of the semiconductor SC1,    -   b) Then under stirring and irradiation of said irradiation        source, a soluble cerium precursor with a degree of oxidation of        +3 is introduced into the suspension that is obtained in step        a),    -   c) Then, under stirring and irradiation of said irradiation        source, a basic agent or acid agent is introduced in such a way        as to bring about the precipitation of the cerium oxide,    -   d) Then, the composition is separated from the suspension of        step c),    -   e) The composition that is obtained in step d) is dried,    -   f) Optionally, the dried composition that is obtained in step e)        is subjected to a heat treatment.

Thus, in step a), while being stirred, a suspension that contains asemiconductor SC1, preferably in the form of nanometric powder, in aliquid mixture that consists of water and/or one or more organiccompounds and at least one metal precursor that is selected from amongan element of groups IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA andVA of the periodic table is prepared, and the suspension is irradiatedby an irradiation source such that at least a portion of the emissionspectrum of said source consists of photons having energies that exceedthe width of the forbidden band of the semiconductor SC1.

The percentage of organic compounds contained in the suspension variesfrom 0 to 100% by volume. The organic compounds are in general primaryor secondary alcohols; in a preferred manner, the organic compounds aremethanol, ethanol, or isopropanol, by themselves or in a mixture.

The metal precursor is introduced into the mixture in the form ofsoluble powder or in solution, preferably in aqueous solution. The metalprecursor is in general based on acetate, acetylacetonate, chloride,nitrate or sulfate. In a preferred manner, the metal precursor is basedon chloride or nitrate.

The metal precursor is selected from among an element of groups IVB, VB,VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of the periodic table,preferably from among those of groups VIIB, VIIIB, IB and IIB of theperiodic table. In a very preferred manner, the precursor is a precursorof platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper,rhenium, or rhodium.

The quantities of the metal precursor that are introduced into thesuspension are selected in such a way that the content of the element(s)M in the metal state is between 0.001 and 20% by weight, and in apreferred manner between 0.01 and 10% by weight in relation to the totalweight of the composition.

The semiconductor SC1 that is introduced in step a) is one of thesemiconductors described previously.

The mixing is preferably carried out at ambient temperature while beingstirred, preferably mechanically or by bubbling.

The mixture is irradiated by a source such that at least a portion ofthe emission spectrum consists of photons having energies that exceedthe width of the forbidden band of the semiconductor that is used.Preferably, the source emits at at least one wavelength range thatexceeds 280 nm, in a very preferred manner 315 nm to 800 nm, whichincludes the UV spectrum and/or the visible spectrum. The radiationsource can be any artificial or natural electromagnetic radiationsource, such as the natural light from the sun, an Hg-type lamp, anXe-type lamp, or an LED-type lamp.

The duration of this step is preferably between 1 minute and 20 hourswhile being irradiated, preferably between 1 minute and 5 hours.

During step a), the metal ions M^(δ+) of the precursor are reduced inthe form of metal particles M° on the surface of the semiconductor SC1under the action of the electrons that are generated by the absorptionof photons by said semiconductor. When the composition is in the form ofa core-shell-type substrate structure, these metal particles will formthe core of the composition according to the invention.

In step b), then under stirring and irradiation of said irradiationsource, a soluble cerium precursor with a degree of oxidation +3 isintroduced into the suspension that is obtained in step a).

The cerium precursor is in general based on chloride, sulfate, acetate,bromide, fluoride, acetylacetonate, nitrate, hydroxide, and carbonate.In a preferred manner, the precursor is cerium chloride or ceriumnitrate.

The cerium precursor can be solubilized before its introduction intowater or a liquid mixture that consists of water and one or more organiccompounds such as primary or secondary alcohols, and in a preferredmanner, methanol, ethanol or isopropanol, by itself or in a mixture.

Optionally, and so as to ensure the solubility of the cerium precursor,an acid agent can be added to the mixture so as to modulate the pH ofthe solution. The acid agent is selected preferably from among theinorganic acids, such as nitric, sulfuric, phosphoric, hydrochloric, orhydrobromic acid, or the organic acids, such as carboxylic or sulfonicacids. The pH of the solution is less than 7, preferably less than 5.

The quantities of the cerium precursor introduced into the suspensionare selected in such a way that the content of cerium oxide, expressedin terms of the element Ce, is between 0.01 and 50% by weight,preferably between 0.5 and 20% by weight, in relation to the totalweight of the composition.

The stirring and irradiation conditions are those described for step a).The stirring and irradiation conditions are preferably identical tothose of step a). The duration of this step is preferably between 1minute and 20 hours, preferably between 1 minute and 5 hours.

In step c), under stirring and irradiation of said irradiation source, abasic agent or acid agent is introduced in such a way as to bring aboutthe precipitation of cerium oxide, in particular Ce₂O₃. Preferably, thepH is modified by adding a basic agent or an acid agent in such a waythat it is within a range of between 9 and 13 after the agent is added.

When a basic agent is introduced, it is preferably selected from amongthe alkaline or alkaline-earth hydroxides, the organic bases such asamines, or ammonia. When an acid agent is introduced, it is preferablyselected from among the inorganic acids such as nitric, sulfuric,phosphoric, hydrochloric, or hydrobromic acid, or the organic acids suchas carboxylic or sulfonic acids.

The stirring and irradiation conditions of step c) are those describedfor step a). The stirring and irradiation conditions are preferablyidentical to those of step a). The duration of this step is preferablybetween 1 minute and 20 hours, preferably between 1 minute and 5 hours.

During step c), the metal ions Ce³⁺ precipitate in the form of a shellof metal oxide Ce₂O₃ on the surfaces of metal particles M deposited instep a), under the action of the basic agent or acid agent that isintroduced. The M/semiconductor SC1 interface promotes the locating ofelectrons that are photogenerated by the photon absorption in saidsemiconductor SC1 on the surfaces of the metal particles M and thusinduces a negative partial charge on the surfaces of said metalparticles M, resulting in the preferred locating of the oxide shellCe₂O₃ on the metal particles because of the electrostatic attractionbetween Ce³⁺ and M^((δ−)).

In step d), the composition is separated from the suspension of step c).The separation can be carried out by filtering or by centrifuging.Preferably, it is carried out by centrifuging. In general, thiscentrifuging is carried out for 10 to 60 minutes at 2000 to 10000 rpm.In a preferred manner, one to three cycles of washing with water arethen carried out.

In step e), the composition that is obtained in step d) is dried. Thedrying is carried out between 30° C. and 200° C., in general for 1 to 48hours, preferably in air. Optionally, this drying can be done underinert atmosphere. The drying can optionally be carried out in an oven ora rotary evaporator. The drying step can optionally be done underpartial vacuum.

According to an embodiment, it is possible to carry out—between steps a)and b)—a step for separation, preferably by centrifuging, an optionalwashing step, and a drying step under the conditions described above.

In an optional manner, the dried composition obtained in step e) issubjected to a heat treatment (step f). The heat treatment is carriedout under a stream of air, nitrogen, hydrogen, or under partial vacuum,in general at a temperature of between 50° C. and 500° C., preferablyfor a duration of between 1 and 16 hours.

Use in Photocatalysis

The invention also relates to the use of the composition according tothe invention as a photocatalyst, and in particular as a photocatalystfor the degradation of organic compounds, such as, for example, formicacid.

The photocatalytic method for degradation of organic compounds, such as,for example, the photocatalytic degradation of formic acid, isimplemented by putting into contact a stream that contains an organiccompound with said composition according to the invention. Then, thecomposition is irradiated by at least one irradiation source thatproduces at least one wavelength that is suitable for the activation ofsaid composition in such a way as to degrade the organic compound, forexample formic acid into hydrogen and into CO₂.

The composition can be used in a photocatalytic method in liquid orgaseous medium. The implementation of the photocatalytic method can bedone in a flow-through fixed bed, in a sweeping fixed bed, or insuspension (also called “slurry” in English terminology). It can also bedone in reactors that are made entirely of glass or that usenon-absorbent optical windows so as to make it possible for theradiation to reach the surface of the solid. The type of technology ofthe reactor for using the solid is generally suitable for a suspension.This type of technology is also called “slurry” in English terminology.The type of technology of the reactor can also be of the solar paneltype with a sweeping or flow-through bed on a porous or non-poroussubstrate. The photocatalyst can also be deposited directly on opticalfibers.

Any source of irradiation that emits at at least one wavelength that issuitable for the activation of said composition, i.e., absorbable by thecomposition, can be used according to the invention. The irradiation ofthe source is therefore such that at least a portion of the emissionspectrum of said source consists of photons with energies that exceedthe width of the forbidden band of the composition according to theinvention. Preferably, the source emits at at least one wavelength rangethat exceeds 280 nm, in a very preferred manner 315 nm to 800 nm, whichincludes the UV spectrum and/or the visible spectrum. The radiationsource can be any source of artificial or natural electromagneticradiation, such as the natural light from the sun, an Hg-type lamp, anXe-type lamp, or an LED-type lamp.

The use of the composition is conditioned by the provision of photonsthat are suited to the photocatalytic system for the reaction inquestion and thereby is not limited to a specific range of pressure ortemperature that is outside of those making it possible to ensure thestability of the product or products. The temperature range employed forthe use of the composition is in general from −10° C. to +200° C., in apreferred manner from 0 to 150° C., and in a very preferred manner from0 to 50° C. The pressure range that is employed for the use of thecomposition is in general from 0.01 MPa to 70 MPa (0.1 to 700 bar), andin a preferred manner from 0.1 MPa to 2 MPa (1 to 20 bar).

The invention is illustrated by the following examples that are not inany case limiting in nature.

EXAMPLES Example 1 Solid A (In Accordance with the Invention)Ce₂O₃/Pt/TiO₂

0.0712 g of H₂PtCl₆,6H₂O (37.5% by mass of metal) is put into 500 ml ofdistilled water. 50 ml of this solution is drawn off and put into aglass double-jacket reactor. 3 ml of methanol and then 250 mg of TiO₂(P25, Degussa™) are then added while being stirred to form a suspension.

The mixture is then left to be stirred and to be exposed to UV radiationfor two hours. The lamp that is used to provide the UV radiation is a125 W mercury vapor lamp HPK™.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The powder that is recovered is finally placed in an oven at 70° C. for24 hours.

The solid A′ Pt/TiO₂ is then obtained. The content of the element Pt ismeasured by atomic emission spectrometry with a plasma source (orinductively-coupled plasma atomic emission spectroscopy “ICP-AES” inEnglish terminology) at 0.93% by mass.

A solution of Ce(NO₃)₃ is prepared by dissolving 0.05 g of Ce(NO₃)₃,6H₂O(Sigma-Aldrich™, 99%) in 50 ml of H₂O, or a concentration of Ce³⁺ of 2.3mmol/L.

The following were introduced into the reactor: 0.10 g of the solid A′,25 ml of distilled water, and finally 25 ml of isopropanol. The systemis purged in the dark under a stream of argon (100 ml/minute) for 2hours. The reactor is thermostated at 25° C. during the entiresynthesis.

The stream of argon is then slowed to 30 ml/minute, and the irradiationof the reaction mixture starts up. The lamp that is used to provide theUV radiation is a 125 W mercury vapor lamp HPK™. Then, 5 ml of thecerium nitrate solution is added to the mixture. The mixture is left for1 hour to be stirred and irradiated. Then, 1 ml of a 30% NH₃ solution isadded. The mixture is again left for 1 hour to be stirred andirradiated.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid A Ce₂O₃/Pt/TiO₂ is then obtained. The content of the elementCe is measured by ICP-AES at 1.7% by mass. By XPS (X-Ray PhotoelectronSpectrometry in English terminology) measurement, a covering of platinumparticles that exceeds 83% and cerium oxide phases with 74% of Ce₂O₃ and26% of CeO₂ is measured. By transmission electron microscopy, a meanthickness of the shell of cerium oxide of 4 nm around metal particles ismeasured.

Example 2 Solid B (In Accordance with the Invention) Ce₂O₃/Pt/TiO₂

0.0710 g of H₂PtCl₆,6H₂O (37.5% by mass of metal) is put into 500 ml ofdistilled water. 50 ml of this solution is drawn off and put into aglass double-jacket reactor. 3 ml of methanol and then 250 mg of TiO₂(P25, Degussa™) are then added while being stirred to form a suspension.

The mixture is then left to be stirred and to be exposed to UV radiationfor two hours. The lamp that is used to provide the UV radiation is a125 W mercury vapor lamp HPK™.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid B′ Pt/TiO₂ is then obtained. The content of the element Pt ismeasured by ICP-AES at 0.92% by mass.

A solution of Ce(NO₃)₃ is prepared by dissolving 0.05 g of Ce(NO₃)₃,6H₂O(Sigma-Aldrich™, 99%) in 50 ml of H₂O, or a concentration of Ce³⁺ of 2.3mmol/L.

The following were introduced into the reactor: 0.10 g of the solid B′,25 ml of distilled water, and finally 25 ml of isopropanol. The systemis purged in the dark under a stream of argon (100 ml/minute) for 2hours. The reactor is thermostated at 25° C. during the entiresynthesis.

The stream of argon is then slowed to 30 ml/minute, and the irradiationof the reaction mixture starts up. The lamp that is used to provide theUV radiation is a 125 W mercury vapor lamp HPK™. Then, 10 ml of thecerium nitrate solution is added to the mixture. The mixture is left for1 hour to be stirred and irradiated. Then, 1 ml of a 30% solution of NH₃is added. The mixture is again left for 1 hour to be stirred andirradiated.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid B Ce₂O₃/Pt/TiO₂ is then obtained. The content of the elementCe is measured by ICP-AES at 3.8% by mass. By XPS measurement, acovering of the platinum particles that exceeds 90% and cerium oxidephases with 77% of Ce₂O₃ and 23% of CeO₂ is measured. By transmissionelectron microscopy, a mean thickness of the shell of cerium oxide of 11nm around metal particles is measured.

Example 3 Solid C (In Accordance with the Invention) Ce₂O₃/Pt/ZnO

0.0710 g of H₂PtCl₆,6H₂O (37.5% by mass of metal) is put into 500 ml ofdistilled water. 50 ml of this solution is drawn off and put into aglass double-jacket reactor. 3 ml of methanol, and then 250 mg of ZnO(Lotus Synthesis™, specific surface area 50 m²/g) are then added whilebeing stirred to form a suspension.

The mixture is then left to be stirred and to be exposed to UV radiationfor two hours. The lamp that is used to provide the UV radiation is a125 W mercury vapor lamp HPK™.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid C′ Pt/ZnO is then obtained. The content of the element Pt ismeasured by ICP-AES at 0.80% by mass.

A solution of Ce(NO₃)₃ is prepared by dissolving 0.05 g of Ce(NO₃)₃,6H₂O(Sigma-Aldrich™, 99%) in 50 ml of H₂O, or a concentration of Ce³⁺ of 2.3mmol/L.

The following were introduced into the reactor: 0.10 g of the solid C′,25 ml of distilled water, and finally 25 ml of isopropanol. The systemis purged in the dark under a stream of argon (100 ml/minute) for 2hours. The reactor is thermostated at 25° C. during the entiresynthesis.

The stream of argon is then slowed to 30 ml/minute, and the irradiationof the reaction mixture starts up. The lamp that is used to provide theUV radiation is a 125 W mercury vapor lamp HPK™. Then, 10 ml of thecerium nitrate solution is added to the mixture. The mixture is left for1 hour to be stirred and irradiated. 1 ml of a 30% NH₃ solution is thenadded. The mixture is again left for 1 hour to be stirred andirradiated.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid C Ce₂O₃/Pt/ZnO is then obtained. The content of the element Ceis measured by ICP-AES at 3.7% by mass. By XPS measurement, a coveringof platinum particles that exceeds 89% and phases of cerium oxides with81% of Ce₂O₃ and 19% of CeO₂ is measured. By transmission electronmicroscopy, a mean thickness of the shell of cerium oxide of 12 nmaround metal particles is measured.

Example 4 Solid D (In Accordance with the Invention) Ce₂O₃/Au/TiO₂

0.0472 g of HAuCl₄,xH₂O (52% by mass of metal, Aldrich™) is put into 500ml of distilled water. 50 ml of this solution is drawn off and put intoa glass double-jacket reactor. 3 ml of methanol and then 250 mg of TiO₂(P25, Degussa™) are then added while being stirred to form a suspension.

The mixture is then left to be stirred and to be exposed to UV radiationfor two hours. The lamp that is used to provide the UV radiation is a125 W mercury vapor lamp HPK™.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

A solid D′ Au/TiO₂ is then obtained. The content of the element Au ismeasured by ICP-AES at 0.95% by mass.

A solution of Ce(NO₃)₃ is prepared by dissolving 0.05 g of Ce(NO₃)₃,6H₂O(Sigma-Aldrich™, 99%) in 50 ml of H₂O, or a concentration of Ce³⁺ of 2.3mmol/L.

The following were introduced into the reactor: 0.10 g of the solid D′,25 ml of distilled water, and finally 25 ml of isopropanol. The systemis purged in the dark under a stream of argon (100 ml/minute) for 2hours. The reactor is thermostated at 25° C. during the entiresynthesis.

The argon stream is then slowed to 30 ml/minute, and the irradiation ofthe reaction mixture starts up. The lamp that is used to provide the UVradiation is a 125 W mercury vapor lamp HPK™. Then, 10 ml of the ceriumnitrate solution is added to the mixture. The mixture is left for 1 hourto be stirred and irradiated. Then, 1 ml of a 30% solution of NH₃ isadded. The mixture is again left for 1 hour to be stirred andirradiated.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid D Ce₂O₃/Au/TiO₂ is then obtained. The content of the elementCe is measured by ICP-AES at 3.6% by mass. By XPS measurement, acovering of gold particles that exceeds 90% and cerium oxide phases with75% of Ce₂O₃ and 25% of CeO₂ is measured. By transmission electronmicroscopy, a mean thickness of the shell of cerium oxide of 14 nmaround metal particles is measured.

Example 5 Solid E (Not in Accordance with the Invention) Ce₂O₃/Pt/TiO₂

0.0710 g of H₂PtCl₆,6H₂O (37.5% by mass of metal, Aldrich™) is put into500 ml of distilled water. 50 ml of this solution is drawn off and putinto a glass double-jacket reactor. 3 ml of methanol and then 250 mg ofTiO₂ (P25, Degussa™) are then added while being stirred to form asuspension.

The mixture is then left to be stirred and to be exposed to UV radiationfor two hours. The lamp that is used to provide the UV radiation is a125 W mercury vapor lamp HPK™.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

A solid E′ Pt/TiO₂ is then obtained. The content of the element Pt ismeasured by ICP-AES at 0.94% by mass.

A solution of Ce(NO₃)₃ is prepared by dissolving 0.05 g of Ce(NO₃)₃,6H₂O(Sigma-Aldrich™, 99%) in 50 ml of H₂O, or a concentration of Ce³⁺ of 2.3mmol/L.

The following were introduced into the reactor: 0.10 g of the solid E′,25 ml of distilled water, and finally 25 ml of isopropanol. The systemis purged in the dark under a stream of argon (100 ml/minute) for 2hours. The reactor is thermostated at 25° C. during the entiresynthesis.

The stream of argon is then slowed to 30 ml/minute. Then, 10 ml of thecerium nitrate solution is added to the mixture. The mixture is left for1 hour to be stirred and without irradiation. Then, 1 ml of a 30%solution of NH₃ is added. The mixture is again left for 1 hour to bestirred and without irradiation.

The mixture is then centrifuged for 10 minutes at 3000 rpm so as torecover the solid. Two cycles of washing with water are then carriedout, with each of the washing cycles being followed by a centrifuging.The recovered powder is finally placed in an oven at 70° C. for 24hours.

The solid E Ce₂O₃/Pt/TiO₂ is then obtained. The content of the elementCe is measured by ICP-AES at 3.8% by mass. By XPS measurement, acovering of the platinum particles on the order of 21% and cerium oxidephases with 76% of Ce₂O₃ and 24% of CeO₂ is measured. By transmissionelectron microscopy, a shell of cerium oxide around metal particles issometimes distinguished without carrying out a measurement of the meanthickness because of the inhomogeneity of the distribution.

Example 6 Solid F (Not in Accordance with the Invention) TiO₂

The solid F is commercial titanium dioxide TiO₂ P25, Degussa™.

Example 7 Evaluation of Solids by Photocatalytic Degradation of FormicAcid

The solids A, B, C, D, E, and F are subjected to a photocatalytic testfor production of dihydrogen by degradation of formic acid in asemi-open Pyrex reactor that is stirred and is equipped with a quartzoptical window and a double jacket to regulate the test temperature.

100 mg of solids is suspended in 60 ml of an aqueous solution of formicacid at 0.5 mol/L. The tests are carried out at 25° C. under atmosphericpressure with a flow rate of argon of 5 ml/minute to entrain thedihydrogen gas that is produced and that is analyzed by gas phasechromatography. The visible UV irradiation source is provided by anXe—Hg lamp (Asahi™, MAX302™). The irradiation power is always kept at100%. The duration of the test is 20 hours.

The photocatalytic activities are expressed in terms of μmol ofdihydrogen that is produced per hour and per gram of photocatalyst. Theresults are recorded in Table 1. The activity values show that thesolids according to the invention systematically offer the bestphotocatalytic performances.

TABLE 1 Performances of the Solids in Initial Activity for theProduction of Dihydrogen by Degradation of Formic Acid InitialPhotocatalyst Activity SC2/M/SC1 (μmol/h/g) Solid A (In Accordance)Ce₂O₃/Pt/TiO₂ 1027 Solid B (In Accordance) Ce₂O₃/Pt/TiO₂ 1232 Solid C(In Accordance) Ce₂O₃/Pt/ZnO 793 Solid D (In Accordance) Ce₂O₃/Au/TiO₂912 Solid E (Not in Accordance) Ce₂O₃/Pt/TiO₂ 234 Solid F (Not inAccordance) TiO₂ 12

The invention claimed is:
 1. A composition comprising a firstsemiconductor SC1, particles comprising one or more element(s) M in themetal state that are an element of groups IVB, VB, VIB, VIIB, VIIIB, IB,IIB, IIIA, IVA or VA of the periodic table, and a second semiconductorSC2 that comprises cerium oxide, with said first semiconductor SC1 beingin direct contact with said particles that comprise one or moreelement(s) M in the metal state, with said particles being in directcontact with said second semiconductor SC2 that comprises cerium oxidein such a way that the second semiconductor SC2 covers at least 50% ofthe total surface of the particles that comprise one or more element(s)M in the metal state, and wherein said first semiconductor SC1 forms asubstrate, said substrate contains on its surface core-shell particles,with said shell being formed by said semiconductor SC2 that comprisescerium oxide, said core being formed by said particles that comprise oneor more element(s) M in the metal state.
 2. The composition according toclaim 1, in which the first semiconductor SC1 is in direct contact withthe second semiconductor SC2.
 3. The composition according to claim 1,in which the cerium oxide consists of Ce₂O₃.
 4. The compositionaccording to claim 1, in which the one or more element(s) M in the metalstate are platinum, palladium, gold, nickel, cobalt, ruthenium, silver,copper, rhenium, or rhodium.
 5. The composition according to claim 1, inwhich the cerium oxide content of the semiconductor SC2, expressed interms of the element Ce, is between 0.01 and 50% by weight in relationto the total weight of the composition.
 6. The composition according toclaim 1, in which the content of the one or more element(s) M in themetal state is between 0.001 and 20% by weight in relation to the totalweight of the composition.
 7. The composition according to claim 1, inwhich said particles that comprise one or more element(s) M in the metalstate are in the form of particles of sizes of between 0.5 nm and 1000nm.
 8. The composition according to claim 1, in the form of nanometricpowder.
 9. The composition according to claim 1, in which thesemiconductor SC1 is TiO₂, Bi₂S₃, Bi₂O₃, Fe₂O₃, ZnO, WO₃, CuO, ZnFe₂O₄,MoS₂, or In(OH)₃.
 10. The composition according to claim 1, in which theshell has a thickness of 1 nm to 1000 nm.
 11. A method for preparationof the composition according to claim 1 comprising: a) preparing understirring a suspension that contains a first semiconductor SC1 in aliquid mixture that consists of water and/or one or more organiccompounds and at least one metal precursor that is selected from amongan element of groups IVB, VB, VIB, VIIB, VIIIB, IB IIB, IIIA, IVA and VAof the periodic table and irradiating the suspension by an irradiationsource such that at least a portion of the emission spectrum of saidsource consists of photons having energies that exceed the width of theforbidden band of the semiconductor SC1, b) under stirring andirradiation of said irradiation source, introducing a soluble ceriumprecursor with a degree of oxidation of +3 into the suspension that isobtained in step a), c) under stirring and irradiation of saidirradiation source, introducing a basic agent or acid agent in such away as to bring about the precipitation of cerium oxide, d) separatingthe composition from the suspension of c), e) drying the compositionthat is obtained in d), f) and optionally, subjecting the driedcomposition that is obtained in e) to a heat treatment.
 12. Thepreparation method according to claim 11, in which the at least onemetal precursor is a precursor of platinum, palladium, god, nickel,cobalt, ruthenium, silver, copper, rhenium, or rhodium.
 13. Thepreparation method according to claim 11, in which in c), the pH isbetween 9 and 13 after the basic agent or acid agent is introduced. 14.A method of photocatalysis, comprising subjecting a feed to be catalyzedto radiation in the presence of a catalyst according to claim 1.